The present invention relates generally to a method and apparatus for measuring volume and/or weight of quantities of viscous materials, and more particularly, to a method and apparatus for measuring and controlling the size of drops of viscous material dispensed from a dispensing system.
There are several types of prior art dispensing machines used for dispensing metered amounts of liquid or paste for a variety of applications. One such application is in the assembly of surface mount printed circuit boards, wherein a large quantity of small dots of adhesive liquid are dispensed in precise, predetermined locations on a circuit board. The dots of adhesive liquid are used for connecting components to the circuit board. Another application of dispensing machines is in dispensing viscous material used for encapsulating integrated circuit chips and/or for under filling flip integrated circuit chips. The dispensing machines are generally required to run continuously to achieve a high throughput, and are also required to achieve a high degree of repeatability (i.e., be able to dispense substantially identical dots with a very small tolerance or variability between dots). The dispensing systems described above include those manufactured by Camelot Systems, Inc., the assignee of the present invention, and distributed under the name CAM/ALOT(copyright).
In one prior art dispensing system, the weight of the dots dispensed by the system is periodically measured by a scale during calibration routines, and the dispensing system includes a feedback system to adjust the quantity of fluid dispensed by the dispensing system, so that the weight of the dots is controlled. There are several drawbacks associated with this prior art system, and these drawbacks are of particular concern in dispensing systems used for dispensing solder paste at precise locations on a circuit board, and in dispensing systems used for under filling flip chips. In these systems, the desired weight of the viscous drops is typically on the order of 0.002 grams. Scales utilized to accurately measure dots of this size are typically rather expensive, and the measured weight of a sample can vary greatly depending on temperature of the air, temperature of the sample, and due to vibration of the scale.
Another problem with the prior art system that measures weight is that for typical dispensing systems, it is more desirable to control the volume of material dispensed than to control the weight of the material dispensed. Although volume can be calculated based on weight when the specific gravity of the dispensing material is known, it has been found that the specific gravity of typical dispensing materials used for encapsulating integrated circuits tends to vary from its specified value after the material has been dispensed. If specific gravity is used to calculate volume based on weight, any variations in the specific gravity of the material dispensed will cause errors in the calculated volume.
A further problem associated with the prior art dispensing systems that use a scale to measure the weight of dispensed drops is that the materials dispensed by these systems are typically hazardous materials that require special precautions for disposal.
Embodiments of the present invention overcome the drawbacks of the prior art dispensing systems described above, and provide a method and apparatus for determining volume and/or weight of drops produced by a dispensing system during a calibration routine of the dispensing system. In embodiments of the present invention, a feedback control system, utilizing the method and apparatus for determining volume and/or weight, is incorporated into a dispensing system to ensure that consistent drops of a predetermined size are dispensed by the dispensing system.
In a first aspect of the present invention, a dispensing system for dispensing material onto a substrate includes a housing, a dispensing apparatus that dispenses a metered quantity of material, and a measuring apparatus. The measuring apparatus includes a bottom plate having a top surface to receive the material dispensed from the dispensing apparatus, a top plate positionable over the bottom plate after the material has been dispensed, and a compressing apparatus that compresses the material between the top and bottom plates. The dispensing system further includes a viewing system, coupled to the housing, that views the compressed material in the measuring apparatus to determine the quantity of material dispensed.
In an alternate version of the first aspect of the invention, the dispensing system further includes a control system that controls the quantity of material dispensed by the dispensing apparatus based on the quantity of material measured by the viewing system, thereby establishing a closed loop system that controls the quantity of material dispensed.
In a second aspect of the present invention, a dispensing system includes a housing, a dispensing apparatus that dispenses a quantity of material, and a measuring apparatus. The measuring apparatus includes a bottom plate having a top surface to receive the material dispensed from the dispensing apparatus, a top plate positionable over the bottom plate after the material has been dispensed, and a compressing apparatus that compresses the material between the top and bottom plates. The dispensing system further includes means for determining the quantity of material dispensed based on characteristics of the material compressed in the measuring apparatus.
In an alternate version of the second aspect, the dispensing system further includes means for controlling the quantity of material dispensed by the dispensing apparatus based on the quantity of material determined by the means for determining, thereby establishing a closed loop system that controls the quantity of material dispensed.
In a third aspect of the present invention, a measuring apparatus for determining a quantity of a viscous material includes a housing, a bottom plate having a top surface to receive the viscous material, a top plate positionable over the bottom plate after the viscous material has been received, a compressing apparatus that compresses the viscous material between the top and bottom plates, and a viewing system that views the compressed viscous material to determine the quantity of viscous material dispensed.
In an alternate version of the third aspect of the invention, the measuring apparatus further includes a gap shim disposed between the top plate and the bottom plate such that the compressed viscous material has a width equal to a width of the gap shim.
Another aspect of the present invention is directed to a method of measuring a quantity of material dispensed from a dispensing apparatus. The method includes steps of dispensing the material on to a bottom plate, disposing a top plate over the bottom plate, compressing the material between the top plate and the bottom plate, and viewing the compressed material through the top plate to determine the quantity of material dispensed.
In a preferred version of the method of measuring a quantity of material, the method further includes steps of comparing the quantity of material dispensed with a desired quantity to generate an error signal indicative of a difference between the desired quantity and the quantity dispensed, and controlling the quantity of material dispensed by the dispensing system based on the error signal.
In a fourth aspect of the present invention, a dispensing system includes a housing, a dispensing apparatus that dispenses a quantity of material and a measuring apparatus. The measuring apparatus includes a platform having a recessed area, a first slide that is placed in the recessed area to receive dispensing material from the dispensing head and a second slide that is disposed above the first slide to compress the dispensing material between the first slide and the second slide. The dispensing system also includes a vacuum head for loading the slides onto the platform and includes a viewing system that views the compressed material.
In one version of the fourth aspect of the invention, the dispensing system includes a controller programmed to determine a delay of the dispensing apparatus and programmed to determine a velocity of the dispensing apparatus with respect to a substrate based in part on the delay determined.